Method for forwarding a message cell stream via a plurality of parallel trunks while adhering to the sequence of the message cells

ABSTRACT

A message cell stream that carries message cells transmitted according to an asynchronous transfer method during the course of virtual connections is forwarded via a plurality of parallel trunks. Successive message cells are thereby cyclically distributed onto the trunks with a fixed temporal offset. Upon reception of the message cells, the trunks are cyclically interrogated in the same run sense with a temporal offset that is shorter than the temporal offset with which successive message cells are distributed onto the trunks.

BACKGROUND OF THE INVENTION

The invention is directed to a method for forwarding a message cellstream via a plurality of parallel trunks, whereby

the message cell stream carries fixed-length message cells in the courseof an asynchronous transfer method,

the transport rate of the message cell stream is higher than thetransport rate of an individual trunk,

successive message cells of the message cell stream are cyclicallydistributed onto the plurality of trunks and forwarded.

Conditioned by technology, the transmission bit rate on the individual,physical transmission links of a transmission system is limited by theprocessing speed of the transmission elements such as, for example,trunks or switching elements that are employed. When a message cellstream whose transport bit rate exceeds the transmission bit rate of anindividual, physical transmission link is to be transmitted, then it canbe transmitted via a plurality of parallel, physical transmission links.

IEEE International Conference on Communications ICC '90, ConferenceRecord, 16-19 April 1990, Atlanta, U.S.A., vol. 2, pp. 771-777 disclosesa transmission system wherein a stream of packets carrying useful datais forwarded via a group of parallel trunks. Successive message cellsare thereby cyclically distributed onto the group of trunks and areforwarded during the same time slot. Due to different lengths, differenttemperature or different line lengths of the trunks in the groups oftrunks or due to fluctuations in the clock frequencies givenplesiochronic switching systems, non-directional deviations in runningtime of the packets can occur. Due to the deviations in running time,errors in the sequence of the packets can occur upon reception of thepackets, as shall be set forth in greater detail later.

SUMMARY OF THE INVENTION

The invention is based on the problem of specifying a method that allowsa forwarding of message cells of a message cell stream via a pluralityof parallel trunks while assuring the sequence of the messagecells--even given the occurrence of non-directional deviations inrunning time on the individual trunks.

In the method initially cited, the problem is resolved in that

the successive message cells are temporally offset relative to oneanother;

the trunks are cyclically interrogated in the same sense upon receptionof the message cells, whereby, given the cyclical interrogation ofsuccessive trunks,

a) the offset is shorter than the offset with which successive messagecells are forwarded, and

b) the chronological spacing of the beginning of two successiveinterrogation cycles is shorter than the duration of the forwarding of amessage cell.

Based on the criterion of the need for transmission capacity for amessage cell stream, the method of the invention enables a flexibleofferability of a plurality of trunks, whereby the sequence of themessage cells of the message cell stream is strictly observed at thetransfer to a following transmission equipment. The beginning of aninterrogation cycle is thereby uncorrelated with the transmission timeslots on the trunks.

A development of the method of the invention provides that an arrivalcharacter is set at the arrival of a message cell from a trunk in areception means, and the arrival character that has been set is resetwhen the trunk is interrogated. What this measure assures is that everymessage cell is transferred only once into a reception equipment.

A further development of the method of the invention provides that thedesignations of message cells that are to be forwarded onto a bundle ofsuccessive trunks are entered into the queue belonging to the trunkwhose designation derives by masking the least significant binarycharacter of the binarily presented queue designation. This measureprovides an especially simple selection of a queue for bundle sizeswhose plurality of trunks can be presented by a whole exponent of themantissa 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIG. 1 an illustration of the inventive forwarding of successive messagecells of a message cell stream via a bundle of four trunks;

FIG. 2 a schematic illustration of the interconnection of threeswitching elements at which the method of the invention can be applied;

FIG. 3 an isochronic (synchronous) forwarding of successive messagecells of a message cell stream via a bundle of four trunks in conformitywith the prior art;

FIG. 4 the occurrence of a possible error of the prior art in thesequence of the message cells in the interrogation of the trunks;

FIG. 5 and FIG. 6 the inventive interrogation of a bundle of trunkshaving a beginning of the interrogation respectively modified comparedto FIG. 1 with reference to the arrival time of the first message cellof the message cell stream;

FIG. 7 the inventive interrogation of two message cell streams suppliedto neighboring inputs of a switching element on respectively fourtrunks;

FIG. 8 the sequentially correct forwarding of message cells receivedaccording to FIG. 7 onto a bundle of four trunks;

FIG. 9 the sequentially correct forwarding of message cells receivedaccording to FIG. 7 onto a bundle of eight trunks.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows 3 switching elements SE1, SE2, SE3 each having respectively16 inputs and eight outputs. The structure and functioning of suchswitching elements are known in and of themselves, for example from IEEEJournal on Selected Areas in Communications, vol. 9, no. 8, Oct. 1991,and therefore need not be set forth in greater detail here. The outputsof the switching element SE1 and of the switching element SE2 areconnected to the inputs of the switching element SE3 via trunks VL1 . .. VL8 or, respectively, VLf . . . VLn. The inputs of the switchingelement SE1 and of the switching element SE2 are connected to inputlines EL1 . . . EL4 . . . EL5 . . . EL8 . . . EL13 . . . EL16 or,respectively, ELk . . . ELn . . . ELw . . . ELz. Let the input lines inturn be connected to the outputs of switching elements that are notshown in greater detail or to other equipment suitable for theforwarding of message cells that can be remotely arranged. The outputlines AL0 . . . AL7 connected to the outputs of the switching elementSE3 can in turn be connected to following switching elements or can leadto remote equipment for forwarding message cells. Below, the input linesEL and the output lines AL can be generally considered as trunks VL. Arespective transmission of message cells on the trunks ensues with aconstant message cell rate according to an asynchronous transfer modeduring the course of virtual connections. Let the message cells therebybe a matter of cells having a fixed length, each of which has a cellheader with, for example, a length of five octets as well as a usefuldata part having a length, for example, of 48 octets. The transmissionof the actual message signals ensues in the useful data part. What arethereby to be understood by message signals are message and text signalsas well as voice or, respectively, image signals in digital form. Amongother things, a message cell header comprises what is referred to as avirtual channel number with which the virtual connection coming intoconsideration on the respective trunk is referenced. During transmissionpauses, moreover, dummy cells corresponding to the message cells aretransmitted.

A message cell stream whose transmission bit rate of, for example, 600Mbit/s exceeds the transport bit rate of a trunk having, for example,150 Mbit/s is transmitted via a plurality of four trunks in the example.

According to one feature of the invention, successive message cells of amessage cell stream are cyclically distributed onto a plurality oftrunks, whereby the message cells are accompanied with a fixed, temporaloffset respectively compared to the immediately preceding message cell.FIG. 1 shows a temporal illustration of the forwarding of successive,continuously numbered message cells of a message cell stream via aplurality of four trunks VLn. Within a cyclical distribution of messagecells onto the trunks, the message cells comprise a constant temporaloffset compared to a respectively preceding message cell that, forexample, may be assumed to amount to the transmission duration of threeoctets. Let the trunks from FIG. 1 be established by the input lines EL1. . . ELA, whereby the message cells referenced with the continuousnumbers 1 and 5 are transmitted on the input line EL1 and the messagecells referenced with the continuous numbers 2, 3, 4, 6, 7, 8 areanalogously transmitted on the input lines EL2 . . . EL4. Let the inputsof a switching element SE be respectively connected to an input line. Anarrival character (arrival flag) is set for every input given thearrival of a message cell. The inputs of a switching element arecyclically interrogated for the presence of set arrival flags. Thecyclical interrogation of the inputs ensues in the same run sense withwhich the message cells are distributed onto the trunks. Given apredetermined run direction of the cyclical interrogation of the inputs,the line location of the input lines has been determined. It is providedaccording to a feature of the invention that the temporal offset withwhich successive inputs are interrogated for set arrival flags isshorter than the offset with which the successive message cells areforwarded. The temporal offset with which successive inputs areinterrogated amounts, for example, to the transmission duration of twooctets. FIG. 1 shows an interrogation referenced SC of inputs to whichthe message cell stream is supplied. The black dots indicate that a setarrival flag was recognized. Further, a respectively recognized arrivalflag is reset. Given the interrogation cycle shown in FIG. 1, themessage cells having the continuous sequence numbers 1, 2, 3, 4 aredetected in the correct sequence. Moreover, let it be pointed out thatthe beginning of the cyclical interrogation is not correlated with thearrival of the message cells. The feature of the invention in accordwherewith the temporal spacing of the beginning of two successiveinterrogation cycles is shorter than the duration for forwarding amessage cell is also related thereto.

FIG. 3 shows the forwarding of consecutively numbered message cells of amessage cell stream via a plurality of four trunks according to theprior art, whereby message cells are isochronically (synchronously)transmitted onto the trunks. As shown by black dots, the sequence of themessage cells is correctly detected in the interrogation SC of thetrunks. Given the forwarding of message cells according to the prior artshown in FIG. 4, let an arbitrary shift of the arrival times of themessage cells on the individual trunks occur due, for example, totemperature differences on the individual trunks or due to differentlengths of the individual trunks. An error in the sequence of therecognized message cells occurs given the time slot of two successiveinterrogations SC1, SC2 shown in FIG. 4 with respect to the arrivaltimes of message cells on the individual trunks. During the firstinterrogation SC1, namely, the message cells having the consecutivesequence numbers 2, 3, 4 are recognized and the message cells having theconsecutive sequence numbers 1, 6, 7, 8 are recognized during the secondinterrogation SC2.

FIG. 5 shows two successive interrogations SC1 and SC2 according to themethod of the invention, whereby the beginning of the firstinterrogation SC1 coincides with the point in time of the arrival of themessage cell having the consecutive sequence number 1. The message cellhaving the consecutive sequence number 1 is thereby recognized duringthe first interrogation. The message cell having the consecutivesequence number 1, on the one hand, is no longer recognized during thesecond interrogation SC2, as indicated by a white dot, since its arrivalflag was already reset during the first interrogation SC1, and, on theother hand, the message cells having the consecutive sequence numbers 2,3, 4 are recognized, the sequence of the message cells having thus beenadhered to. As may be seen, the message cells are recognized in thecorrect sequence of their consecutive sequence numbers in this case aswell.

FIG. 6 shows two successive interrogations SC1 and SC2 according to theinventive method, whereby the beginning of the first interrogation SC 1ensues later by the transmission duration of a few octets than the pointin time of the arrival of the message cell having the consecutivesequence number 1. During the first interrogation SC1, the message cellshaving the consecutive sequence numbers 1 and 2 are recognized, and themessage cells having the consecutive sequence numbers 3 and 4 arerecognized during the second interrogation SC2. As may be seen, themessage cells are recognized in the correct sequence of theirconsecutive sequence numbers.

FIG. 7 shows a message cell stream referenced with consecutive sequencenumbers and a message cell stream referenced with upper case letters inan alphabetic sequence that are respectively transmitted on four trunks.Let these two message cell streams comprise a temporal offset relativeto one another with respect to the arrival of the message cell havingthe consecutive sequence number 1 and be supplied to successivelyinterrogated inputs of a switching element. Let the message cell streamhaving the message cells referenced with consecutive sequence numbers besupplied to the switching element SE1 from FIG. 2 on the input lines EL1. . . EL4 and let the message stream having the message cells referencedwith uppercase letters be supplied to this switching element on theinput lines EL5 . . . EL8. As identified by black dots, the messagecells having the designation 1, A, B, C, D are recognized during thefirst interrogation SC1. The message cells having the designation 2, 3,4, E, F, G are recognized during the second interrogation. As may beseen, the message cells of each message cell stream are recognized inthe correct sequence. The recognization of successive message cells of amessage cell stream during different interrogations is insignificant foran asynchronous transfer mode. It may also be seen in the exemplaryembodiment of FIG. 7 that the temporal offset in the interrogationamounts to the chronological duration for the transmission of one octetin the one instance and to the chronological duration for thetransmission of two octets in another instance in continuousalternation. This alternation in the offset is based thereon that oneoutput is serviced after the interrogation of two inputs given aswitching element that comprises a plurality of inputs that is twice asgreat as the plurality of outputs, as, for example, in FIG. 2. Thetemporal offset in the interrogation of the trunks according to FIG. 7is thereby always smaller than the offset with which successive messagecells are forwarded.

The outputs of a switching element are each respectively connected to atrunk leading in a predetermined direction. A queue organized as FIFO isallocated to every output of a switching element. The designations ofmessage cells destined for forwarding of the appertaining trunk areentered into a respective queue in the sequence of their recognitionduring the interrogation of the inputs. A plurality of trunks leading inone direction forms a directional bundle. The designations of messagecells provided for forwarding on a direction bundle are entered in onlyone queue. The designation of a queue allocated to a directional bundlecan ensue by masking the least significant bits (LSB) of the binarilycoded designation of the queues. What is thereby to be understood bymasking is that a binary character is set to the value zero regardlessof its actual status. When, for example, the output lines AL0 . . . AL3and the output lines AL4 . . . AL7 thus each respectively form adirectional bundle, then the queue having the designation 000₂ =000₁₀ isallocated to the directional bundle formed with the output lines AL0 . .. AL3 and the queue having the designation 100₂ =4₁₀ is allocated to thedirectional bundle formed with the output lines AL4 . . . AL7. Forforwarding, the message cells are placed onto the appertaining outputline in the sequence of their designations in the queue. When themessage cells are to be forwarded onto a directional bundle, the messagecells are cyclically output onto the output lines of the directionalbundle in the sequence of their designations in the appertaining queue.

FIG. 8 shows the forwarding of message cells received according to FIG.7 onto a directional bundle having four trunks, whereby the transportrate on each of the four trunks in FIG. 8 is twice as high as that onone of the trunks in FIG. 7.

FIG. 9 shows the forwarding of the message cells received according toFIG. 7 onto a directional bundle having eight trunks, whereby the trunksin FIG. 7 and in FIG. 9 each comprise the respectively same transportrate.

The invention is not limited to the particular details of the methoddepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described method withoutdeparting from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A method for forwarding a message cell stream viaa plurality of parallel trunks, the message cell stream carryingfixed-length message cells during an asynchronous transfer mode, atransport rate of the message cell stream being higher than a transportrate of an individual trunk of the plurality of parallel trunks,successive message cells of the message cell stream being cyclicallydistributed onto the plurality of trunks and being forwarded, comprisingthe steps of:temporally offsetting the successive message cells relativeto one another; cyclically distributing the successive message cellsonto trunks of the plurality of parallel trunks in a predeterminedmanner; cyclically interrogating the trunks of the plurality of paralleltrunks in said predetermined manner upon reception of the message cells,interrogation of the trunks also being offset, whereby, given thecyclical interrogation of successive trunks of the plurality of paralleltrunks,a) the offset of the interrogation is shorter than an offset withwhich successive message cells are forwarded, and b) chronologicalspacing of beginning of two successive interrogation cycles is shorterthan a period of time of the forwarding of a message cell.
 2. The methodaccording to claim 1, wherein the successive message cells are forwardedwith a temporal offset having a fixed chronological duration.
 3. Themethod according to claim 1, wherein the temporal offset with whichsuccessive message cells are forwarded amounts to a fraction of a periodof time of the forwarding of a message cell.
 4. The method according toclaim 3, wherein the fraction is equal to a fractional portion which isequal to a ration of one trunk to a total number of trunks of theplurality of parallel trunks.
 5. The method according to claim 3,wherein the fraction is smaller than a fractional portion, which isequal to a ration of one trunk to a total number of trunks of theplurality of parallel trunks.
 6. The method according to claim 1,wherein trunks of the plurality of parallel trunks are coupled to aswitching element having a central memory and wherein, upon reception ofmessage cells in the switching element, the trunks are interrogated witha temporal offset that is equal to the temporal spacing with whichmessage cells from successively interrogated trunks are written into thecentral memory in the switching element.
 7. The method according toclaim 1, wherein upon arrival of a message cell in a reception equipmentfrom a trunk, an arrival flag is set and wherein the arrival flag thatwas set is reset when the trunk is interrogated.
 8. The method accordingto claim 1, wherein trunks of the plurality of parallel trunks arecoupled to a switching element, and wherein, for forwarding messagecells from the switching element onto a bundle of trunks, designationsof these message cells are entered in a queue associated with thebundle, and these message cells are cyclically forwarded onto the trunksof said bundle based on a criterion of a sequence of the designations inthe queue.
 9. The method according to claim 8, the designations ofmessage cells that are to be forwarded onto a bundle of successivetrunks are entered into a queue belonging to the bundle of successivetrunks, a designation of the bundle of successive trunks being providedwithout taking into consideration the least significant binary characterof the binarily coded queue designation.